Zonal isolation refers to the isolation of a subterranean formation or zone, which serves as a source of a natural resource such as gas, oil, or water, from other subterranean zones. To achieve zonal isolation of a subterranean zone, a well bore is typically drilled down to the subterranean zone while circulating a drilling fluid through the wellbore. After the drilling is terminated, a string of pipe, e.g., casing, is run in the wellbore. Primary cementing is then usually performed whereby a sealant composition, e.g., a cement slurry, is pumped down through the string of pipe and into the annulus between the string of pipe and the walls of the wellbore to allow the cement slurry to set into an impermeable cement column and thereby seal the annulus. Subsequent secondary cementing operations may also be performed. One example of a secondary cementing operation is squeeze cementing whereby a cement slurry is forced under pressure to areas of lost integrity in the annulus to seal off those areas.
Unfortunately, long term zonal isolation is often compromised due to various circumstances. For example, during a transition phase in which a cement slurry changes from a true hydraulic fluid to a highly viscous mass showing some solid characteristics, it is desirable for the cement slurry to transmit hydrostatic pressure to prevent fluid (e.g., gas or water) from flowing from the subterranean zone into the slurry. However, the pressure exerted on the cement slurry by the fluid in the subterranean zone often exceeds the hydrostatic pressure of the cement slurry. When then happens, the fluid initially migrates into and through the cement slurry, forming flow channels therein that undesirably permit further migration of the fluid after the cement composition sets. As such, zonal isolation of the subterranean formation can be lost. Some conventional cement slurries have relatively long transition times that can exacerbate this problem.
Zonal isolation is also commonly compromised by cyclical changes in wellbore and formation pressures and temperatures during the life of the well. The piping in the wellbore to which the cement column is attached and the cement column itself may expand and contract due to the pressure and temperature changes. As such, the cement column is subjected to mechanical stress such as tensile, compressive, or shear stress. Moreover, the cement column may experience impacts and shocks generated by subsequent drilling and other well operations. Unfortunately, conventional cement suffers from the drawback of being brittle and fragile and thus often cannot sustain such stress. Thus, fractures through which fluids can undesirably flow may develop in the cement column. Further, a microannulus may form between the cement column and the piping or the subterranean zone.
Cement compositions such as foamed cement compositions and sealant compositions having improved elasticity have been developed that are more likely to provide for long term zonal isolation than traditional cement compositions. However, those more recently developed compositions suffer from the drawback of having complex designs that require several additives to achieve desired bulk properties. The cost of using the additives in the amounts required to seal a wellbore with a single composition containing such additives is typically very high. A need therefore exists to develop a less expensive method for improving zonal isolation and thus extending the life of the well.